This invention relates generally to semiconductor structures designed to produce a collimated output having a high power level and more particularly to semiconductor structures incorporating a lens element therein to collimate the output light and expand the output aperture.
In numerous applications, collimated beams of high power laser output are desired such as communication lasers, high energy laser systems and orbital power transfer systems. In previous semiconductor laser devices, a limiting factor on the ability to produce an output beam of high power has been the limit imposed by the maximum power density allowed at the emitting facet of the laser. If the semiconductor laser should be operated so that the power emitted from the laser exceeded the maximum power density at the emitting facet of the laser, the semiconductor laser structure itself could be damaged. In order to increase the available output power levels, of semiconductor laser devices, the surface area of the emitting facet may be increased such that a larger amount of power could be output while not exceeding the maximum power density level at the emitting facet. One method of increasing the surface area of the emitting facet is by increasing the width of the stripe, but for strip widths much greater than 5 .mu.m the modes tend to breakup and filamentation dominates. This effect produces a wider beam divergence than one would expect for the respective stripe width because while the filaments each operate in their own single mode, the ensemble of filaments that make up the beam are incoherent. The filamentation in broad stripe devices can be controlled, however, by adding optical elements to the cavity. The optical elements can be designed to form a stable resonator cavity or an unstable resonator cavity. While there has been some experimental success with stable resonator designs at low power levels, unstable resonator designs may theoretically be scaled to even higher power levels. However, additional optical elements must be fabricated and aligned for such cavities which adds to the complexity of the design while making it nonmonolithic.
An additional problem with utilizing semiconductor lasers in many applications is the need to collimate the output of the laser. In many instances, separate collimation elements must be utilized to collimate the slow axis and the fast axis of the semiconductor laser in order to maintain fundamental mode operation.
In an attempt to decrease the use of distinct collimation elements, previous devices have attempted to integrate the optical elements upon the substrate forming the laser. Such prior art devices attempted to direct or form the beam by making deep etches into the semiconductor structure. These etches would typically be made to a depth, below that at which the optical power was circulating. Thus, for a typical semiconductor laser structure, the etch would be made such that it was deeper than the quantum well and its associated confinement layers. Such deep etches are designed to refract the propagating wave. A problem associated with using the semiconductor surface to refract the light when forming a beam is the roughness of the etched surface. With reference to GaAs semiconductor layers an example, the optical index of GaAs is approximately 3.5 while the surface flatness required for a smooth optical lens is a variation of .lambda.100 or less. For a waveguide propagating light having a wavelength of 800 nm, the wavelength in the GaAs material would be (800 nm/3.5) or 228 nm. Thus, the surface flatness required for such a refracting deep etch surface is +/- (22.8 nm/100) or +/-22.8 Angstrom. The fabrication of such a smooth surface has not yet been achieved by current processing techniques.
Therefore, it would be desirable for a semiconductor structure to serve both as a waveguide for the transmitted lightwave as well as a lens element for the slow axis of the laser resonator. Furthermore, it would be desirable if such an integral lens element could be formed with a minimal number of additional processing steps. It would be desirable if such a semiconductor waveguide structure could be used as a conventional one-dimensional optic element that would allow the collimation of expanding modes, as well as being utilized in the design of both conventional and unique laser resonators.